This invention relates to plastic closures and more particularly to pilfer-resistant plastic closures of the type commonly used on beverage bottles and containers for food, pharmaceuticals, and industrial products.
The prior art of pilfer-resistant plastic closures for beverage bottles and the like has been highly developed. In general, such closures have a pilfer-resistant (tamper-resistant) band with an internal bead or internal lugs adapted to engage an annular shoulder on the neck of the bottle when the closure has been applied to the bottle. When the bottle is opened, the band breaks away (partially or entirely) from the body of the closure, so that even if the closure is reapplied to the bottle, the fact that the bottle has been opened is evident. To permit the pilfer-resistant band to break away, the closure is formed with a line of weakness or with a plurality of circumferentially spaced frangible bridges that break when the closure is removed.
Plastic closures are commonly manufactured by injection or compression molding. In one type of molding apparatus, mold parts move transversely or radially with respect to the longitudinal axis of the closure to release the closure from the mold. In another type of molding apparatus, which, in general, is simpler, mold parts move axially (along the longitudinal axis of the closure) to release the closure from the mold. The present invention is concerned with closures produced by the latter type of apparatus.
Premature breakage of the bridges that connect a tamper-resistant band to the body of a plastic closure has been a serious problem, particularly in closures that are manufactured by molding apparatus in which the closures are released from the mold by axial movement of mold parts. Such movement tends to stretch the bridges, which may cause breakage of at least some of the bridges even before the closure is applied to a bottle. Bridge breakage also occurs prematurely during closure application, when the bridges are deformed to force the internal bead of the pilfer-resistant band over the annular shoulder on the neck of the bottle.
Attempts have been made to reduce premature bridge breakage by employing more massive (stronger) bridges, but if the bridges are strong enough to prevent premature breakage, the bridges may not break at all when the closure is removed from the bottle, and the tamper-resistant band may actually stretch and be removed from the bottle when the closure is removed, without breaking the bridges. Furthermore, massive bridges require much greater force to break the bridges, and greater effort to remove the closure from the bottle.
Other attempts to solve the problem of bridge breakage during closure application do not rely on more massive bridges, but instead use thin, readily breakable bridges together with abutments that prevent collapse of the bridges and/or twisting of the bridges during application of a closure to a bottle. However, such techniques complicate the configuration of the mold and do not solve the problem of premature bridge breakage when closures are released from molding apparatus by axial movement of mold parts. One of the attempts to solve that problem, without employing unduly massive bridges, relies upon an axially moving stripper ring that engages surfaces at opposite ends of the bridges in an effort to prevent stretching of the bridges during stripping of closures from the molding apparatus. The stripper ring has notches in its internal surface for forming bridges that are located outward of the internal surfaces of the sidewall and tamper-resistant band, and for providing axial flow paths for molten plastic from the sidewall of the closure to the band. While such stripper rings assist in preventing bridge breakage during stripping of closures from the molding apparatus, it has been discovered that in volume production some of the bridges are nevertheless stretched, elongated, and weakened, causing such bridges to break prematurely. This problem occurs at random and is difficult or impossible to control.